1. Field of the Invention
The invention relates to metal working and, specifically, to a method and apparatus for producing external conical threads at end portions of steel rods or wire of the type used as reinforcements in pre-stressed concrete structures.
2. Prior Art of the Invention
Tapered thread connections are known to provide for quick engagement of multi-threaded male and female connector portions since as few as one or two turns may be sufficient for complete and firm engagement of a large number of threads of both the male and the female part of the thread connection.
Hence, conical threads are particularly suitable as fastening threads when, during tightening, the threaded bolt and/or the threaded nut can be turned only with great effort, or should not be displaced much in axial direction. A well-known use of conical threads is for physical connection or "splicing" of steel reinforcements in pre-stressed concrete by providing the ends of the reinforcing rods or wires with male, i.e. external, conical threads and connecting ends of such reinforcements by means of end pieces, connectors, or nuts each having at least one female, i.e. internal, conical threads, e.g. a connector nut having two such female threads at opposing ends. Such connectors are disclosed, for example, in U.S. Pat. Nos. 3,415,522 and 3,850,535 and are sold commercially by Fox-Howlett Industries of Berkley, Cal.
According to the art, male conical threads at the ends of steel reinforcements are produced by cutting with a thread cutting apparatus having two mutually opposed chasers displaceable in radial direction. An example of such an apparatus is disclosed in European Published Application No. 97 745 (Erico Product, Inc.).
However, thread cutting is known to reduce the strength of a metal structure because the grain structure, structural continuity, or grain flow will be interrupted by a cut thread, and because a notch effect will occur in the core area of the thread's undercut so as to reduce the load-bearing cross-section of the thread. A further disadvantage of prior art thread cutting for producing tapered threads is that it requires great skill, a rather delicate apparatus, and too much time.
Generally, prior art thread-forming methods by non-cutting techniques may involve pressing or rolling; such methods are limited, however, for practical reasons by the magnitude of the forces required for deformation of the material in which the threads are to be made. For example, while cylindrical threads of good quality have been obtained by cold-rolling of steel rods or wires, such a method can be operated with relatively short rollers only and requires roller displacement in axial direction when longer threads are to be obtained. Evidently, axial roller displacement is not practical when non-cylindrical, i.e. conical, threads are to be made and rolling methods are not assumed to be applicable to normal production of male conical threads on high-tensile steel structures.
For the same reason, cold-pressing of male conical threads is restricted to structures or metals, e.g. aluminum alloys or the like, that are substantially softer than high-tensile steel structures of the type suitable for concrete reinforcement by pre-stressing.
Now, with the substantial and increasing importance of pre-stressing methods in concrete constructions such as bridges, wide-span roofing, or shell structures and the like there is a substantial and increasing need for a simple method and tool that permits to form external tapered threads at a construction site since, while the female connectors are normally factory-produced and supplied to the construction site ready for use, steel wires or rods may not, or not always, be provided ready for use at the construction site taking into account that external threads are sensitive to accidental damage during transportation and handling, and that production of male tapered threads may be required at a construction site because of specific dimensional requirements or the like needs.
Further, prior art tapered male threads produced by thread cutting suffer from all the above-mentioned disadvantages of cut threads, i.e. lower mechanical strength and less corrosion resistance.